When I was a young man I remember seeing some mechanical, educational computer toys that absolutely fascinated me. I vividly recall the Hasbro “Think-A-Tron” (1960) that answered questions on little mini-punch cards, and the plastic Digi-Comp mechanical calculator (1967).

Several people have built recreations of the Digi-Comp. I decided to do so too. I had access to some scrap acrylic plastic and a laser cutter. I designed my version (I called it Gravi-Comp just to distinguish it from the commercial re-creations) to be totally modular. The idea there is that I wanted to be able to test each module individually, and modify and replace each module when necessary, rather than have to modify the entire device. I had a lot of help from several colleagues, including Kirk Olynyk, Nathan Brown, and Chris O’Dowd.

Some video colleagues made a very short video that gives you a rough idea of what the device is like in action.

Click here to see a short video clip

The resulting machine has about 150 separate modules, most of which are either 4 in. x 2 in., or 4 in. by 4 in. in size. The device ended up being much larger than I thought it’d be. Overall, it’s 12 feet long by 3 feet wide. When tilted on its stand, the device stands over 7 feet tall.

These mechanical computers are programmed by manually setting a series of switches (about 32 of them). At the top of the device is a hopper that holds 1.25 in. diameter acrylic balls. The balls are released one at a time. As each ball drops, it passes through switches, toggling some of them to the left or the right. The final answer to the programmed problem is stored as a binary number in seven of the switches.

I’m not exactly sure what I’ll do with the Gravi-Comp now that it’s finished, but it was a lot of fun to build it on weekend mornings.